Recently, synthetic fiber, synthetic resin film or such film laminated with insulating paper, like kraft paper, is applied as insulation material of low power loss or low dielectric loss for use with an ultrahigh voltage, oil filled cable intended for large capacity transmission line.
However, synthetic resin film, for example, has the drawbacks that it has low chemical resistance to insulation oil conducted through the above-mentioned cable. The film, when impregnated with oil, swells and tends to increase oil flow resistance, and readily gives rise to buckling and creases when the cable is bent. Where the synthetic resin film is applied in combination with insulating paper, then the combined mass does indeed produce a smaller increase in physical resistance to oil flow resulting from swelling, but is still accompanied with the drawbacks that when swelling, the combined mass obstructs radial oil flow within the cable insulation and shows a noticeable polar effect on an impulse breakdown voltage.
It is accordingly an object of this invention to provide an insulating paper sheet having excellent electric and mechanical properties and great ease of handling.
Another object of the invention is to provide insulating paper for a power cable which is free of the above-mentioned drawbacks accompanying the conventional art insulating paper.